This invention relates to transducers and more particularly to magnetostrictive transducers using permanent magnets to provide a magnetic bias field to lanthanide series magnetostrictive drive elements.
Magnetic polarization of magnetostrictive materials is required in order to provide linear frequency operation and to utilize the maximum strain capabilities of the material. In the absence of biasing the output signal frequency is twice the input drive frequency due to the fact that in any magnetostrictive material the strain is either positive or negative regardless of the polarity of the drive signal. Therefore, the absence of biasing causes the transducer's electromechanical coupling coefficient and its resulting efficiency to be very low.
Magnetostrictive materials such as nickel and Permendur materials were commonly used as driving elements in transducers prior to the development of piezoelectrically polarized titanates. Prior to 1946, magnetostrictive ring transducers were not area or mass loaded, instead their ac excitation and dc polarization coils were toroidally wound on laminated ring stacks or scroll-wound continuous strips of nickel or Permendur. Permanent magnets were rarely used to series bias magnetostrictive ring or loop structures having uniform cross-sectional area. Those ring and loop structures that were biased with permanent magnets, usually Alnico-5 or sintered iron-oxide magnets, used magnets of cross-sectional areas greater than that of the magnetostrictive material. These particular magnets were the best available but were easily demagnitized by alternating signal flux densities. The magnets of these prior state of the art art designs did not require special shaping to concentrate the flux distribution through the magnetostrictive element because the permeability of the magnet was much lower than that of the magnetostrictive element. The air gap between the magnet and the magnetostrictive element had to be minimized which meant that the magnet was typically mounted adjacent to the element, and the excitation coil would then encompass the magnet and the magnetostrictive element. The magnets, therefore, would have to be copper-clad in order to shield them from being demagnetized by the alternating signal flux. Unfortunately, even large rings of these prior art magnetostrictive materials could not provide displacements great enough to produce useful acoustic power at the lower end of the audio frequency spectrum.
In recent years, much interest in magnetostrictively driven transducers is being shown since the development of the lanthanide series of magnetostrictive materials employing Samarium, Terbium, Dysprosium. One of the best of these lanthanide series materials is Terfenol D (Tb.sub.0.3 Dy.sub.0.7 Fe.sub.2). These new alloys offer very high magnetostrictive strain capabilities thereby allowing much greater acoustic power output at lower operating frequencies. Unfortunately, these new materials have very low permeabilities and hence are difficult to bias. The prior art method of biasing comprises superimposing an AC drive field onto a DC biasing field using appropriate passive blocking components to separate the AC drive source and the DC power supply. Both sources energize a common solenoid encompassing the magnetostrictive element. The element is commonly fabricated in bar shape with grain orientation along the length of the bar to maximize the strain per unit magnetomotive force applied to the bar. This common solenoid technique for biasing produces heating of the solenoid and the magnetostrictive bar which reduces the power obtainable from the transducer.
It is therefore the object of this invention to eliminate the need for a direct current bias field by utilizing permanent magnets to provide the required biasing of the magnetostrictive elements. Features of the invention include the reduction of coil winding losses, reduction of wiring complexity and the elimination of coupling components which isolate the AC drive from the DC drive resulting in significant simplification of the power driver design.